This invention relates to an improved laminated wood flooring for truck trailers and containers. Fiber reinforced polymer composite materials are used in conjunction with edge-glued laminated wood members to provide improved mechanical properties, moisture protection, fatigue resistance and light weight to the resulting composite boards for use in flooring of vehicular trailers.
Conventional wood flooring for over-the-road truck trailers and containers is normally manufactured with hardwoods such as oak, maple, birch, beech, etc. The green lumber used as a starting material in such manufacture is suitably dried in special drying chambers under controlled conditions. The dried lumber is then sawed into strips of rectangular cross-section and defective portions are eliminated by cross cutting the strips. During the crosscutting process, "hooks" are formed at the ends of the lumber strips. The relatively defect-free lumber strips are coated on their vertical sides or edges with an adhesive such as urea-melamine formaldehyde or polyvinyl acetate. The uncured edge-glued lumber strips are then assembled on a conveyor by placing them side to side and behind other strips, which were previously assembled. Applying heat and edge pressure to large sections of the assembled lumber strips cures the adhesive thus forming a unitary panel. During the assembly of the lumber strips, "hook joints" are formed at each end of every lumber strip. These joints are simple mechanical couplings between the mating hook ends of opposing lumber strips without significant adhesive bonding at the joint itself. Often times, due to imperfect assembly, a readily visible gap is formed at the hook joints, which can be seen from the top and bottom surfaces of the finished laminated wood floor.
The cured laminated wood is cut to a desired length (up to about 60 feet) and width (about 6 to 18 inches ) to form boards. The boards are then planed to a desired thickness and shiplaps and crusher beads are machined on its sides. A shiplap is a rectangular projecting lip running along the length on each side of a floor board. The crusher bead is a small semi-circular projection running along the length on each side of a board and placed over or below a lip. When the floor boards are assembled in a trailer such that the side edges of corresponding boards are squeezed together, the shiplaps of adjacent boards overlap to form a seam. The crusher beads provide spacing between adjacent boards and help in preventing buckling of the boards due to expansion on absorption of water. A wood putty is applied at the hook joints on the top and bottom surfaces of the boards to fill any resident gaps. Finally, the underside of the floor boards is coated with a polymeric substance termed as "undercoating" to provide moisture protection. The finished floor boards are assembled into a kit of about eight boards for installation in trailers. Normally, a kit consists of two boards with special shiplaps so that they will fit along the road and curb sides of a trailer. The other boards may be identical in design and they are placed between the road and curb side boards. In some trailers, a metallic component such as a hat-channel may be placed between any two adjacent boards. The metallic component becomes part of the floor area. The boards adjacent to the hat-channel have shiplaps designed to mate with the flanges of the metallic component. All the boards are supported by thin-walled cross-members of I, C or hat sections, each having an upper flange, which span the width of the trailer and are spaced along the length of the trailer. Each floor board is secured to the cross-members by screws extending through the thickness of the board and the upper flanges of the cross-members.
Hardwood-based laminated wood flooring is popularly used in truck trailers since it offers many advantages. The surface characteristics of hardwoods such as high wear resistance and slip resistance are most desirable. The strength and stiffness of the flooring is important for efficient and safe transfer of the applied loads to the cross-members of the trailer. The shock resistance of wood is useful to withstand any sudden dropping of heavy cargo on the floor. Nail holding capability and ability to absorb small amounts of water, oil or grease without significantly affecting slip resistance are yet additional favorable properties of hardwood flooring.
Although the conventional wood flooring has many desirable features, it also suffers from certain disadvantages. For example, water from the roads is known to leak into trailers through the gaps of the hook joints that exist in the flooring. The reasons for the water leaks are believed to be the capillary action of the gaps and the tendency of the end grain of wood to absorb and store water. Although the undercoating is supposed to provide a barrier to the path of water, it may not properly cover larger gaps thus exposing them to moisture. Wetting and drying cycles can degrade the undercoating leading to its cracking and peeling away from the wood. Bending of the floor between two adjacent cross-members due to any applied load on the top of the floor also has a tendency to open the hook joints and enlarge the gaps.
A lift truck is often used on the trailer floor to load and unload cargo. The dynamic action of a moving lift truck placing heavy cargo on the trailer floor creates severe stress concentration in the flooring and some of the cross-members. A very large amount of the weight of the lift truck and that of the cargo is transferred to the flooring through the wheels of the front axle of the lift truck due to the momentary raising of the rear axle when the lift truck is dynamically placing a heavy cargo on the floor. The effect of repeated lift truck operation on the conventional wood floor causes considerable fatigue damage including: delamination of the edge glued lumber strips near the hook joints leading to the "pop-out" of the lumber strips; crack initiation and propagation in wood on the underside of the floor due to tensile stresses; and cracking of edge glue lines due to shearing, transverse bending and twisting of the floor. The combination of moisture attack and fatigue damage to the wood floor affects its performance thus necessitating its repair or replacement. In some cases, catastrophic structural failure of the trailer floor system may occur leading to the unacceptable injury to working personnel and damage to machinery.
To alleviate the above-mentioned problems, novel fiber reinforced composite wood flooring was designed, tested and refined to be an improvement over conventional wood flooring. This new composite wood flooring consists of conventional laminated wood member with an underlay of fiber reinforced plastic (FRP) bonded to the wood member. The top surface of the composite wood flooring is essentially the same as that of the conventional wood flooring. Since the FRP is impervious to the passage of water, it completely seals the bottom of the wood member and solves the problem of leaky hook joints. The fiber reinforcement improves the mechanical properties of the flooring and therefore the thickness of the laminated wood can be reduced. Thus, thinner and lighter composite wood flooring can be produced with equivalent strength when compared to thicker conventional wood flooring. Since the reinforcement provides an excellent barrier to the "pop-out" of lumber strips, the fatigue resistance of the composite wood flooring can be improved over that of the conventional wood flooring.
Technologists are constantly trying to find ways to improve the mechanical properties, reduce weight and improve moisture resistance of wood flooring. Fouquet, U.S. Pat. No. 5,143,418 describes the use of composite plywood panels as flooring in truck trailers. The plywood was composed of veneers of wood with a majority of the veneers oriented with the wood grain along the longitudinal direction while the remaining veneers were oriented with the wood grain along the transverse direction. The top and bottom surfaces of the plywood panels were overlaid with resin impregnated cellulose sheets for providing moisture and slip resistance. Clearly, Fouquet has not considered a floor design involving the FRP to provide higher strength and moisture protection.
Another area of related art is the use of FRP to improve the mechanical properties of structural wood members, such as beams, columns and trusses. Theakston (Canadian Agricultural Engineering, January 1965, Pages 17-19) has discussed the use of glass fibers and epoxy resin to reinforce laminated timber beams and arches. Triantafillou and Deskovic (Journal of Structural Engineering, Vol. 118, No. 5, May 1992, Pages 1270-1284) have published test results on the reinforcement of structural wood beams by adhesively bonding prestressed carbon fiber based FRP panels using epoxy adhesive. Thus the concept of reinforcing structural wood members (especially beams) with FRP has been known for several decades. Tingley, U.S. Pat. No. 5,362,545 describes the use of a resorcinol adhesive to bond certain special composite panels to glue-laminated wood beams (Glulams). The special composite panels containing aramid (Kevlar.RTM.) fiber reinforcement are abraded by sanding prior to bonding. The sanding process makes the panel "hair up" due to Kevlar and helps to obtain improved bonding with wood. The Tingley patent teaches the utility of Kevlar in FRP panels to improve the bond strength of the FRP to wood while using a resorcinol adhesive.
The above-referenced patents and publications have not addressed the construction and related benefits of reinforced laminated wood flooring for use in truck trailers. The advantages of reinforcing the bottom side and disadvantages of reinforcing the top side of the laminated wood boards are not disclosed in these references. The publications do not discuss the remedies for the problems associated with the conventional wood flooring such as water leakage through the hook joints and fatigue damage due to lift truck traffic on the floor. There is no discussion in the publications regarding the type of reinforcements and resins that are suited for the fabrication of reinforced composite wood flooring. For example, the question of whether fiber reinforcement along the width (transverse) direction of the floor is advantageous is not addressed in these publications. A reinforced composite wood flooring construction such as that provided by the present invention which is suitable for lift truck movement and also for carrying cargo in a trailer has never before been invented.
Methods of manufacturing the reinforced composite wood boards to construct the improved flooring have never been considered before. The manufacturing process needs to be able to produce the composite wood boards at a fairly high speed to meet the demands of volume and cost effectiveness. The conventional laminated wood boards are typically manufactured at a rate of about 1500 to 2000 sq ft/hr using one set of machinery. To be competitive, composite wood boards need to be produced at a similar production rate. Since dissimilar materials with mismatch of physical properties are bonded together to manufacture composite wood boards, precaution has to be taken to ensure the flatness of the board after the bonding is completed. The adhesive used to bond the FRP and wood member should not excessively leak under the bonding pressure. Squeeze-out of adhesive can contaminate the surfaces of the substrates requiring additional clean-up operation. The bond between the FRP and wood member needs to be strong enough to resist the stress concentration around the hook joints and also be fatigue and moisture resistant.